This invention relates to the extraction and reutilization of surfactants from oil, water and chemical emulsions that are produced as a result of enhanced oil recovery operations.
Surfactant flooding has become one of the more promising enhanced oil recovery techniques for recovering oil after water flooding. Generally, the method of surfactant flooding employs the injection of a surfactant in an aqueous solution or in an emulsion to sweep through the formation and recover oil. This may be followed by a polymer solution for mobility control and improved sweep efficiency. It is believed that surfactants are effective because of their ability to reduce the interfacial tension between crude oil and water. This reduction of interfacial tension permits the creation of various oil, water and surfactant emulsions in the formation. Thus, the produced liquids of surfactant flooding generally contain very stable oil, water and surfactant emulsions.
Because of the relative amounts of oil and water produced in chemical flooding, the produced emulsions will usually be oil-in-water emulsions. Upon settling, especially in the early stages of a chemical flood, such emulsions may change to the water-in-oil emulsions which are usually encountered in primary petroleum production. Conventional surfactant extraction and emulsion breaking techniques which work on water-in-oil emulsions may be ineffective with oil-in-water emulsions.
Considerable quantities of surfactants may be entrained in the produced emulsions. This prevents the effective reutilization of the produced surfactant in the continuing surfactant flooding process. Because surfactants are expensive additives for chemical floods, commercial economics require that surfactant costs be minimized by reutilizing a substantial quantity of surfactant that is injected into the formation. The methods which are available to separate the produced emulsions fail to extract and concentrate the produced surfactant sufficiently for reutilization of the surfactant.
One method of breaking an oil-in-water emulsion is described in U.S. Pat. No. 4,029,570 where additional formation brine is added to the produced emulsion to produce an oil phase and a water phase. However, such brine addition treatments will in many cases not work on produced fluids. One reason is the tendency of formation rock to selectively adsorb high equivalent weight sulfonates, leaving lower equivalent weight sulfonates behind, rendering the surfactant highly hydrophilic. Such processes will not sufficiently extract and concentrate the surfactant to allow reutilization. The surfactant is often left highly diluted in a relatively large volume of oil-in-water emulsion unsuitable for reutilization.
U.S. Pat. No. 3,637,521 discloses an emulsion breaking process comprising the steps of (1) adding an acid to lower the pH of the emulsion to approximately 5.0 and (2) adding an alkylamine containing 1 to 5 nitrogen atoms and from 2 to 12 carbon atoms. U.S. Pat. No. 3,687,845 describes a process wherein oil-in-water emulsions are treated by the addition of a high molecular weight, water-soluble polymer such as the polyvinyl aromatics of benzene such as polystyrene, polyvinyl toluene and several acrylamide compounds.
Another method which has been tried to break oil-in-water emulsions is the use of additional surface active agents. U.S. Pat. No. 4,261,812 discloses a method which employs an additional surface active agent having an average equivalent weight higher than the equivalent weight of the surface active agents in the produced emulsion, preferably an equivalent weight of about 400 to 600. U.S. Pat. No. 4,073,344 mentions the use of dimethylbenzene ammonium chloride, trimethyl ammonium chloride and alkyl benzene sulfonates for the same purpose. The use of additional solubilizers to break oil-in-water emulsions was promoted by U.S. Pat. No. 4,277,352 which suggests adding materials commonly used as solubilizers in surfactant flooding to the produced emulsion.
A different approach to breaking emulsions and recovering surfactant is disclosed in U.S. Pat. No. 4,216,079. This method employs the addition of brine and a low molecular weight alcohol having one to five carbon atoms, preferably isopropyl alcohol, to break the emulsion into three phases with the majority of the surfactant being partitioned into a middle phase. A chief drawback to this particular method is that the conditions necessary to form the three phase region can be difficult to identify, and those conditions may change rapidly because of the varying concentrations of components of the produced emulsions. Substantially greater experimentation is required to fine tune the method of U.S. Pat. No. 4,216,079 to be in the three phase region than for an emulsion breaking method which breaks the emulsion into two phases.